Camshaft adjustment mechanisms, also known as variable camshaft timing devices, are used in the automotive field to control the timing of the camshaft with respect to the crankshaft. By advancing or retarding the angular position of the camshaft, the timing of the camshaft can be adjusted while the engine is operating to account for parameters such as engine load and speed. The use of a camshaft adjustment mechanism allows the valve lift event timing of an engine to be changed, which can help increase fuel efficiency, engine performance, and idle stability, while reducing emissions.
In known camshaft adjustment mechanisms, a hydraulic operating fluid such as engine oil is selectively provided to cavities formed in the camshaft adjustment mechanism to vary the angular position of the camshaft relative to the crankshaft, which results in the camshaft timing being advanced or retarded. An oil control valve is generally used to control the flow of oil to advance, retard, or hold the camshaft position. However, in some cases the supply of engine oil to the camshaft adjustment mechanism is cut off when the engine is stopped, and the camshaft position cannot be maintained during this time. In situations where the engine has been stopped for an extended period of time before being restarted, the cavities of the camshaft adjustment mechanism usually have very low or no oil pressure. The camshaft adjustment mechanism is thus in an uncontrolled and unstable condition. When the engine is started, the camshaft adjustment mechanism violently rotates between the most advanced and most retarded positions until sufficient oil pressure is supplied to the internal cavities. This results in large amounts of noise during engine start and increased wear and damage to the camshaft adjustment mechanism.
To address this problem, various locking mechanisms have been used to lock the camshaft adjustment mechanism from rotation during engine start. One such locking mechanism utilizes a single locking pin to maintain the camshaft adjustment mechanism in an intermediate position when the engine is stopped. However, there is a tradeoff between the locking pin clearance and locking reliability. To minimize the locking pin clearance so that the pin does not move around in a corresponding opening during engine start to produce unwanted noise and vibration, the opening must have approximately the same size as the pin. The tradeoff is that with a small clearance, it is difficult to ensure that the pin fully engages with the corresponding opening in the locked position. Where the pin is only partially engaged with the opening, locking may fail to occur and the additional wear on the pin can cause durability problems. In other known locking mechanisms where two locking pins are used, the pins are actuated by oil flow through the same oil channels used to advance and retard the camshaft position, and controlled by the same proportional oil control valve as the camshaft adjustment mechanism. The downside to these locking mechanisms is that locking and unlocking can only occur reliably under certain conditions. Because the advancing and retarding operation is generally done at high speeds, actuation of the locking pins through the same oil channels used to advance and retard the camshaft position must also be done at those high speeds. This makes it more difficult to control the timing for locking the camshaft adjustment mechanism, decreases locking reliability, and increases the wear on the pins. Therefore, a need exists for a camshaft adjustment mechanism having a locking mechanism that can reliably lock the camshaft adjustment mechanism at a desired engine start position under various operating conditions, while minimizing noise, vibration, and harshness.